Sandpack Patterns

Quick Start Template

Most examples are single-file. Copy this and modify:

<Sandpack>

`js import { useState } from 'react';

export default function Example() { const [value, setValue] = useState(0);

return ( <button onClick={() => setValue(value + 1)}> Clicked {value} times </button> ); } `

</Sandpack>

File Naming

Pattern Usage

Main file (no prefix)

```js src/FileName.js

Supporting files

```js src/File.js active

Active file (reference pages)

```js src/data.js hidden

Hidden files

```css

CSS styles

```json package.json

External dependencies

Critical: Main file must have export default
.

Line Highlighting

```js {2-4}
function Example() {
  // Lines 2-4
  // will be
  // highlighted
  return null;
}

Code References (numbered callouts)

```js [[1, 4, "age"], [2, 4, "setAge"]]
// Creates numbered markers pointing to "age" and "setAge" on line 4

Expected Errors (intentionally broken examples)

```js {expectedErrors: {'react-compiler': [7]}}
// Line 7 shows as expected error

Multi-File Example

<Sandpack>

```js src/App.js
import Gallery from './Gallery.js';

export default function App() {
  return <Gallery />;
}

export default function Gallery() {
  return <h1>Gallery</h1>;
}

h1 { color: purple; }

External Dependencies

<Sandpack>

```js
import { useImmer } from 'use-immer';
// ...

{
  "dependencies": {
    "immer": "1.7.3",
    "use-immer": "0.5.1",
    "react": "latest",
    "react-dom": "latest",
    "react-scripts": "latest"
  }
}

Code Style in Sandpack (Required)

Sandpack examples are held to strict code style standards:

- Function declarations for components (not arrows)

- e
 for event parameters

- Single quotes in JSX

- const
 unless reassignment needed

- Spaces in destructuring: ({ props })
 not ({props})

- Two-line createRoot: separate declaration and render call

- Multiline if statements: always use braces

Don't Create Hydration Mismatches

Sandpack examples must produce the same output on server and client:

// 🚫 This will cause hydration warnings
export default function App() {
  const isClient = typeof window !== 'undefined';
  return <div>{isClient ? 'Client' : 'Server'}</div>;
}

Use Ref for Non-Rendered State

// 🚫 Don't trigger re-renders for non-visual state
const [mounted, setMounted] = useState(false);
useEffect(() => { setMounted(true); }, []);

// ✅ Use ref instead
const mounted = useRef(false);
useEffect(() => { mounted.current = true; }, []);

forwardRef and memo Patterns

forwardRef - Use Named Function

// ✅ Named function for DevTools display name
const MyInput = forwardRef(function MyInput(props, ref) {
  return <input {...props} ref={ref} />;
});

// 🚫 Anonymous loses name
const MyInput = forwardRef((props, ref) => { ... });

memo - Use Named Function

// ✅ Preserves component name
const Greeting = memo(function Greeting({ name }) {
  return <h1>Hello, {name}</h1>;
});

Line Length

- Prose: ~80 characters

- Code: ~60-70 characters

- Break long lines to avoid horizontal scrolling

Anti-Patterns

Pattern
Problem
Fix

const Comp = () => {}

Not standard
function Comp() {}

onClick={(event) => ...}

Conflicts with global
onClick={(e) => ...}

useState
 for non-rendered values
Re-renders
Use useRef

Reading window
 during render
Hydration mismatch
Check in useEffect

Single-line if without braces
Harder to debug
Use multiline with braces

Chained createRoot().render()

Less clear
Two statements

//...
 without space
Inconsistent
// ...
 with space

Tabs
Inconsistent
2 spaces

ReactDOM.render

Deprecated
Use createRoot

Fake package names
Confusing
Use './your-storage-layer'

PropsWithChildren

Outdated
children?: ReactNode

Missing key
 in lists
Warnings
Always include key

Additional Code Quality Rules

Always Include Keys in Lists

// ✅ Correct
{items.map(item => <li key={item.id}>{item.name}</li>)}

// 🚫 Wrong - missing key
{items.map(item => <li>{item.name}</li>)}

Use Realistic Import Paths

// ✅ Correct - descriptive path
import { fetchData } from './your-data-layer';

// 🚫 Wrong - looks like a real npm package
import { fetchData } from 'cool-data-lib';

Console.log Labels

// ✅ Correct - labeled for clarity
console.log('User:', user);
console.log('Component Stack:', errorInfo.componentStack);

// 🚫 Wrong - unlabeled
console.log(user);

Keep Delays Reasonable

// ✅ Correct - 1-1.5 seconds
setTimeout(() => setLoading(false), 1000);

// 🚫 Wrong - too long, feels sluggish
setTimeout(() => setLoading(false), 3000);

Updating Line Highlights

When modifying code in examples with line highlights ({2-4}
), always update the highlight line numbers to match the new code. Incorrect line numbers cause rendering crashes.

File Name Conventions

- Capitalize file names for component files: Gallery.js
 not gallery.js

- After initially explaining files are in src/
, refer to files by name only: Gallery.js
 not src/Gallery.js

Naming Conventions in Code

Components: PascalCase

- Profile
, Avatar
, TodoList
, PackingList

State variables: Destructured pattern

- const [count, setCount] = useState(0)

- Booleans: [isOnline, setIsOnline]
, [isPacked, setIsPacked]

- Status strings: 'typing'
, 'submitting'
, 'success'
, 'error'

Event handlers:

- handleClick
, handleSubmit
, handleAddTask

Props for callbacks:

- onClick
, onChange
, onAddTask
, onSelect

Custom Hooks:

- useOnlineStatus
, useChatRoom
, useFormInput

Reducer actions:

- Past tense: 'added'
, 'changed'
, 'deleted'

- Snake_case compounds: 'changed_selection'
, 'sent_message'

Updater functions: Single letter

- setCount(n => n + 1)

Pedagogical Code Markers

Wrong vs right code:

// 🔴 Avoid: redundant state and unnecessary Effect
// ✅ Good: calculated during rendering

Console.log for lifecycle teaching:

console.log('✅ Connecting...');
console.log('❌ Disconnected.');

Server/Client Labeling

// Server Component
async function Notes() {
  const notes = await db.notes.getAll();
}

// Client Component
"use client"
export default function Expandable({children}) {
  const [expanded, setExpanded] = useState(false);
}

Bundle Size Annotations

import marked from 'marked'; // 35.9K (11.2K gzipped)
import sanitizeHtml from 'sanitize-html'; // 206K (63.3K gzipped)

Sandpack Example Guidelines

Package.json Rules

Include package.json when:

- Using external npm packages (immer, remarkable, leaflet, toastify-js, etc.)

- Demonstrating experimental/canary React features

- Requiring specific React versions (react: beta
, react: 19.0.0-rc-*
)

Omit package.json when:

- Example uses only built-in React features

- No external dependencies needed

- Teaching basic hooks, state, or components

Always mark package.json as hidden:

```json package.json hidden
{
  "dependencies": {
    "react": "latest",
    "react-dom": "latest",
    "react-scripts": "latest",
    "immer": "1.7.3"
  }
}

**Version conventions:**
- Use `"latest"` for stable features
- Use exact versions only when compatibility requires it
- Include minimal dependencies (just what the example needs)

### Hidden File Patterns

**Always hide these file types:**

| File Type | Reason |
|-----------|--------|
| `package.json` | Configuration not the teaching point |
| `sandbox.config.json` | Sandbox setup is boilerplate |
| `public/index.html` | HTML structure not the focus |
| `src/data.js` | When it contains sample/mock data |
| `src/api.js` | When showing API usage, not implementation |
| `src/styles.css` | When styling is not the lesson |
| `src/router.js` | Supporting infrastructure |
| `src/actions.js` | Server action implementation details |

**Rationale:**
- Reduces cognitive load
- Keeps focus on the primary concept
- Creates cleaner, more focused examples

**Example:**
```mdx
```js src/data.js hidden
export const items = [
  { id: 1, name: 'Item 1' },
  { id: 2, name: 'Item 2' },
];

### Active File Patterns

**Mark as active when:**
- File contains the primary teaching concept
- Learner should focus on this code first
- Component demonstrates the hook/pattern being taught

**Effect of the `active` marker:**
- Sets initial editor tab focus when Sandpack loads
- Signals "this is what you should study"
- Works with hidden files to create focused examples

**Most common active file:** `src/index.js` or `src/App.js`

**Example:**
```mdx
```js src/App.js active
// This file will be focused when example loads
export default function App() {
  // ...
}

### File Structure Guidelines

| Scenario | Structure | Reason |
|----------|-----------|--------|
| Basic hook usage | Single file | Simple, focused |
| Teaching imports | 2-3 files | Shows modularity |
| Context patterns | 4-5 files | Realistic structure |
| Complex state | 3+ files | Separation of concerns |

**Single File Examples (70% of cases):**
- Use for simple concepts
- 50-200 lines typical
- Best for: Counter, text inputs, basic hooks

**Multi-File Examples (30% of cases):**
- Use when teaching modularity/imports
- Use for context patterns (4-5 files)
- Use when component is reused

**File Naming:**
- Main component: `App.js` (capitalized)
- Component files: `Gallery.js`, `Button.js` (capitalized)
- Data files: `data.js` (lowercase)
- Utility files: `utils.js` (lowercase)
- Context files: `TasksContext.js` (named after what they provide)

### Code Size Limits

- Single file: **<200 lines**
- Multi-file total: **150-300 lines**
- Main component: **100-150 lines**
- Supporting files: **20-40 lines each**

### CSS Guidelines

**Always:**
- Include minimal CSS for demo interactivity
- Use semantic class names (`.panel`, `.button-primary`, `.panel-dark`)
- Support light/dark themes when showing UI concepts
- Keep CSS visible (never hidden)

**Size Guidelines:**
- Minimal (5-10 lines): Basic button styling, spacing
- Medium (15-30 lines): Panel styling, form layouts
- Complex (40+ lines): Only for layout-focused examples